1. Field of Invention
The present invention relates generally to the field of mobile telephone communication systems. More specifically, the present invention relates to improving downlink signal quality in the presence of signal fading.
2. Background of the Invention
FIG. 1 is a schematic diagram illustrating a conventional mobile telephone system 102. As used throughout this specification, the term xe2x80x9cmobile telephone systemsxe2x80x9d includes cellular telephone systems, and the term xe2x80x9cmobile telephonesxe2x80x9d includes cellular telephones. A base station 104 controls communication with one or more mobile telephones, for example, mobile telephone 105. Base station 104 transmits information over multiple radio frequency (RF) signals 107 using radios (not shown). The multiple RF signals are combined into a single RF signal for transmission to the one or more mobile telephones. Prior to transmission, a multi-channel power amplifier (MCPA) 108 amplifies the combined RF signal for transmission to the one or more mobile telephones. The signal is transmitted through a receiver/transmitter 110 to a receiver/transmitter on the mobile telephones, for example, receiver/transmitter 112 on mobile telephone 105.
An inherent problem with mobile telephone communication systems is the presence of fading in the communication channel between the cell site and the mobile telephone. Because there is only one antenna, any fading of the channel, affects data quality. For example, fading can cause bit errors in digital communication systems which seriously degrade the quality of the transmitted signals such as digitally-encoded voice signals.
One method to avoid the effects of such fading is to decorrelate the effects of the fading on the transmitted bit stream. A conventional decorrelation technique is to divide the communication data for a particular user into multiple slots as done, for example, in system using time division multiple access (TDMA) transmission. According to one well-known TDMA system, TIA/EIA 136, there are six slots available to transmit the digital data corresponding to the voice traffic on the channel. Decorrelation of the fading channel is accomplished by interleaving the communication among the available slots. Interleaving is the process of sending the digital data for each user in multiple time slots. For example, the digital data corresponding to the voice traffic for a user might be sent in time slots 1 and 4. By distributing the digital voice data in this fashion, it is hoped that a fade that occurs in one time slot will not occur in the other time slot.
At slow speeds, however, the fades are long in duration relative to the time of a slot. In a single antenna system, therefore, a single fade is likely to affect both time slots that are dedicated to the user. Thus, a more robust decorrelation scheme is required for low speed data transmission.
One such scheme is known as antenna hopping. Antenna hopping uses the property that transmission across different antennas, spaced far enough apart, have uncorrelated fading statistics. Antenna hopping requires two antennas. Such a system is illustrated in FIG. 2. Referring to FIG. 2, a second antenna 204 is added to system 102 for communicating with the one or more mobile telephones. In addition, a switch 202 is added to system 102 to switch between antenna 110 and antenna 204. In conventional antenna hopping systems, data for a particular user for time slot 1 is transmitted out of antenna 104 while the data for that user for time slot 4 is transmitted out of antenna 204. Thus, any fading of the transmitted signal is uncorrelated. That is, the odds that both time slots are experiencing a fade are practically zero because the antennas are spaced sufficiently far apart to reduce the likelihood that fading affects both antennas to near zero. One such antenna hopping system is described in Olofsson, H., et al., xe2x80x9cTransmitter Diversity with Antenna Hopping for Wireless Communication Systems,xe2x80x9d 1997 IEEE 4th Vehicular Technology Conference, vol. 3 at 1743-7, which is incorporated by reference herein in its entirety.
One problem with conventional antenna hopping systems is that there is a possibility that one or the other of the antennas is experiencing a fade. In that case, no switching should take place. That is, there is no intelligence to control when the antennas should be switched to improve communication.
A more significant problem with conventional antenna hopping-systems is that the power out of MCPA 108 is sufficiently high that arcing occurs when switch 202 switches between antenna 110 and antenna 204. This arcing manifests itself as RF spikes which can damage system components and cause failures. Conventional systems can only switch about 10 watts without arcing. However, systems to support mobile communications have to be able to switch in excess of 2000 watts.
The present invention solves the forgoing problems in the art by intelligently switching between antennas when the mobile telephone determines that downlink signal quality has sufficiently degraded. To make this assessment, the mobile telephone makes a measurement of downlink signal quality. In one preferred embodiment the mobile telephone compares this measured signal quality to a predetermined threshold. If the value of the measured downlink signal quality is below the predetermined threshold, the mobile telephone sends an instruction to the base station to switch between antennas. In an alternative embodiment of the present invention, the mobile telephone transmits the measurement itself to the base station. The base station compares the measured downlink signal quality to the predetermined threshold. If the value of the measured downlink signal quality is below the predetermined threshold, the base station switches between antennas. That is, the base station generates the instruction to switch between the antennas based on the measured downlink signal quality that it receives. Alternatively, the base station generates the instruction to switch between the antennas without feedback from the mobile telephone. The instruction that is generated by the base station or the mobile telephone is alternatively referred to herein as a switching instruction.
In a preferred embodiment of the present invention, more than two antennas are available for the base station to use to communicate with a mobile telephone. In this embodiment, the antennas can be switched in a predetermined sequence or a specific antenna can be identified for use.
The switching is preferably controlled by a synchronization signal. The synchronization signal controls the timing of the switches so as to avoid the arcing that would otherwise occur in conventional antenna hopping systems. In a preferred embodiment of the present invention, a continuous low-power signal is applied to an amplifier to prevent repeated cutting off and turning on of the amplifier, thereby preserving its useful life. The synchronization signal can be generated by the base station with or without feedback from the mobile telephone, or as an instruction from the mobile telephone.
Thus, one object of the present invention is to improve communication between a base station and a mobile telephone by reducing the effect of fading on that communication.
Another object of the present invention is to eliminate arcing that occurs in conventional antenna hopping systems.
Another object of the present invention is to synchronize switching between antennas using a synchronization signal.
Another object of the present invention is to allow the use of more than two antennas in an antenna hopping system.
Another object of the present invention is to reduce deterioration of system components, which would otherwise reduce the useful lives of the system components.
These and other objects of the present invention are described in greater detail in the detailed description of the invention, the appended drawings and the attached claims.